Thank you for your advise.

But I didn't understand why WinCC can not read data from other PLC's over Modbus.

Suppose that I add a new channel of ModbusTCPIP.chn type into tag management of WinCC. If I add correct IP and port of PLC's on the ModbusTCP network then I shall be permitted to add tags to specific Modbus Adresses. Should't I?

The only problem is the limitation on the PLC type in the Modbus channel config dialog box (whch is limitted to three Modicon PLC's only). Did you mean this part may cause a problem? (our third party PLC is Hitachi)

Best regards